BMX Promotes Dual Phosphorylation of Tyrosine Kinases

Bone marrow tyrosine kinase gene on chromosome X (BMX) is a broadly expressed nonreceptor tyrosine kinase that is overexpressed in several cancer types. BMX has been shown to regulate the activity of several signaling proteins and has roles in angiogenesis, proliferation, differentiation, motility, and apoptosis, but the direct downstream targets of BMX are unknown. Chen and colleagues determined the optimal substrate motif of BMX using a positional scanning peptide library with a tyrosine at the central position and one of the 20 naturally occurring amino acids, phosphothreonine, or phosphotyrosine at fixed positions upstream or downstream of the central tyrosine. Biotinylated peptides were included in a kinase assay with BMX and radiolabeled ATP, and radiolabel incorporation was measured upon peptide capture with an avidin-coated membrane. BMX had a strong preference for a motif with a phosphotyrosine at position −1, suggesting that BMX requires a priming phosphorylation event. A screen of a phosphorylation site database for potential BMX substrates with adjacent phosphotyrosines revealed that many receptor and nonreceptor tyrosine kinases contained this motif within their kinase domains. BMX promoted dual tyrosine phosphorylation of several of these tyrosine kinases, including MET, FGFR1, focal adhesion kinase (FAK), and the insulin receptor, which was frequently required for full kinase activation. Because BMX and FAK are known to physically interact, the authors focused on FAK and showed that BMX only phosphorylated FAK at tyrosine 577 after SRC-dependent phosphorylation of tyrosine 576. Moreover, BMX-dependent dual phosphorylation at these residues was required for FAK substrate phosphorylation and FAK-dependent cell migration. Through its preference for priming phosphotyrosines, BMX may potentiate the activity of multiple tyrosine kinases simultaneously, suggesting that BMX inhibitors currently in development may be broadly effective in cancers dependent on increased signaling by BMX substrates.

Chen S, Jiang X, Gewinner CA, Asara JM, Simon NI, Cai C, et al. Tyrosine kinase BMX phosphorylates phosphotyrosine-primed motif mediating the activation of multiple receptor tyrosine kinases. Sci Signal 2013;6:ra40.